Epoxy resin compositions each containing an epoxy resin are widely used in laminated-plate resin materials, electric insulating materials, semiconductor encapsulating materials, fiber-reinforced composite materials, coating materials, molding materials, adhesive materials, etc. in view of the excellent physical properties such as heat resistance, low moisture absorption, and the like.
In recent years, in these various applications, particularly in the field of advanced materials, durability performance characterized by heat resistance has been required to be further improved. In particular, with miniaturization and higher integration of electronic materials, resins exhibiting high heat resistance have been required for bonding a substrate, a circuit, and a module. In particular, higher functional epoxy resins having heat dissipation and low thermal expansibility in addition to high heat resistance are required for normally operating semiconductor modules which easily become a high temperature.
For example, tetrafunctional naphthalene-based epoxy resins represented by a structural formula (1) below
are known as epoxy resin materials which can comply with the requirements for high degrees of heat resistance, low moisture absorption, and low thermal expansibility (Patent Literature 1).
In comparison with general phenol novolac-type epoxy resins, the tetrafunctional naphthalene-based epoxy resins have a naphthalene skeleton having high heat resistance and high hydrophobicity, a high crosslink density due to tetrafunctionality, and a molecular structure with excellent symmetry, and thus cured products thereof exhibit very excellent heat resistance. However, higher performance has recently been required for heat resistance, and further improvement has been required. Further, since the tetrafunctional naphthalene-based epoxy resins have low solubility in solvents, the characteristics of the cured products have been not sufficiently exhibited in, for example, manufacture of electronic circuit boards.
It is considered to be effective as a method for improving heat resistance that in the tetrafunctional naphthalene-based epoxy resins, naphthalene rings are directly bonded to each other, not bonded through a methylene structure, because the methylene structure is relatively weak against high temperatures. There is described an epoxy resin having a bi(dihydroxynaphthalene) structure in which a dihydroxynaphthalene dimer has a direct single bond without containing a methylene structure (Patent Literatures 3 to 6). The positions of hydroxyl groups in dihydroxynaphthalene and the bond position in a dimer are important factors which influence the physical properties of an epoxy resin using the resin, such as the softening point, solvent solubility, the heat resistance of cured products thereof, and the like. However, any one of Patent Literatures 2 to 5 does not specify the positions of hydroxyl groups in dihydroxynaphthalene and the bond position in a dimer, and does not describe specific compounds.